Carbon fiber composite materials in which matrix resins such as thermoset resins, thermoplastic resins and the like are reinforced with carbon fibers have excellent modulus of tensile elasticity and tensile strength, and thus they have been utilized in sports, leisure, aerospace, and in addition, in blades for wind power generation and the like.
Mechanical characteristics, such as strength, modulus of elasticity and the like of carbon fiber composite materials are largely affected with affinity and adhesive strength between carbon fibers and matrix resins. Accordingly, oxidation treatments such as introductions of functional oxygen groups onto surfaces of carbon fibers are usually performed. As oxidation treatments, chemical oxidation, electrolytic oxidation, gas phase oxidation and the like in liquid phases have been known.
However, when a matrix resin is a resin of low-polarity, such as polypropylene, an oxidation treatment on surfaces of carbon fibers does not sufficiently increase adhesive strength, and thus polypropylene should also be treated with maleic acid graft and the like, which results in a higher cost.
On the other hand, in addition to an oxidation treatment on surfaces of carbon fibers, it has been attempted to further coat the surface of the carbon fibers with a resin. For example, Patent Document 1 discloses the example to improve adhesiveness of carbon fibers to an unsaturated polyester resin by using a surface treatment agent for carbon fibers, which consists of an epoxy resin; an alkylene oxide adduct of monocyclic or polycyclic phenols; and a polyester condensate of an unsaturated dibasic acid or an ester forming derivative thereof and an alkylene oxide adduct of vinylphenols.